Mutations in the p53 tumor suppressor gene are the most common genetic alterations found in diverse types of human cancer, including the primary malignant brain tumor, glioblastoma multiforme. To estimate the frequency of p53 mutations in human brain tumors, we screened 120 human primary brain tumors (59 astrocytic; 61 nonastrocytic) by the polymerase chain reaction-single-strand conformation polymorphism technique. Six astrocytic tumors (one anaplastic astrocytoma and five glioblastoma multiforme) were found to have putative p53 mutations. Direct sequencing of polymerase chain reaction-amplified deoxyribonucleic acid from these six tumors confirmed the presence of different point mutations in the conserved regions of the p53 gene. Allelic losses on chromosome 17p were detected in four (67%) of the six tumors with p53 mutations. p53 mutations were not detected in any of the 61 nonastrocytic brain tumors. Also, polymerase chain reaction-single-strand conformation polymorphism analysis of 74 leukocyte deoxyribonucleic acid samples from patients with astrocytic and nonastrocytic brain tumors failed to detect any germ-line p53 mutations. We conclude from these findings that p53 gene mutations in brain neoplasms are primarily limited to tumors of astrocytic origin and that the p53 gene mutations in sporadic astrocytomas are somatic in origin (i.e., nonprenatally determined).